Non-volatile memory cell having metal nano-particles for trapping charges and fabrication thereof

ABSTRACT

A non-volatile memory cell is described. The non-volatile memory cell comprises a substrate, a charge-trapping layer, a gate and a source/drain. The charge-trapping layer comprises an insulating layer and metal nano-particles contained therein, wherein the metal nano-particles are formed with thermal dissociation of an oxide of the same metal. The gate is disposed on the charge-trapping layer, and the source/drain is located in the substrate beside the gate.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method forfabricating the same. More particularly, the present invention relatesto a non-volatile memory cell and a method for fabricating the same.

2. Description of the Related Art

Non-volatile memory devices that use charge-trapping mechanisms havebeen widely studied. An earlier trapping-type non-volatile memory deviceis the mirror-bit SONOS memory as described in U.S. Pat. No. 5,768,192,which uses a nitride layer disposed between two oxide layers for chargestorage. Since silicon nitride is an insulating material and the chargestrapped in the nitride trapping layer are localized, two bits can bestored in one memory cell as a hot charge injection mechanism used forprogramming.

Recently, a category of trapping-type non-volatile memory that usesisolated conductive nano-particles for charge storage has been proposed.For example, U.S. Pat. No. 6,342,716 B1 and U.S. Pat. No. 6,413,819disclose a trapping-type non-volatile memory that uses isolated dotelements for charge storage. U.S. Pat. No. 6,165,842 discloses a methodthat forms silicon nano-crystals for charge storage by etching apolysilicon layer. U.S. Pat. No. 6,297,095 B1 discloses a method thatforms silicon nano-crystals for charge storage with controlled LPCVD orUHVCVD. Besides, U.S. Pat. No. 6,303,516 B1 discloses a method thatforms fine metal particles for charge storage by using anantigen-antibody pair binding method. In addition, U.S. Pat. Nos.5,783,263 and 5,876,480 disclose other methods for making nano-crystals.U.S. Pat. No. 5,783,263 discloses a method that forms metalnano-particles by using carbon arc discharge on a graphite rod packedwith the same metal, alloy, or oxide of metal or alloy. U.S. Pat. No.5,876,480 discloses a method that forms metal nano-particles by reducingions of the same metal with some catalytic metal ions that are carriedby vesicles in a dispersion system.

To well control the electrical properties of a trapping-typenon-volatile memory that uses nano-particles for charge storage, gooduniformity in density of the nano-particles is required. Unfortunately,the nano-particles made with any one of the aforementioned methods inthe prior art have poor uniformity, and the electrical properties of thenon-volatile memory cannot be well controlled.

SUMMARY OF INVENTION

In view of the forgoing, this invention provides a non-volatile memorycell that contains metal nano-particles with better uniformity forcharge storage.

This invention also provides a method for fabricating a non-volatilememory cell The method uses thermal dissociation for forming metalnano-particles to improve the uniformity of the same.

The non-volatile memory cell of this invention comprises a substrate, acharge-trapping layer, a gate and a source/drain. The charge-trappinglayer comprises an insulating layer and metal nano-particles containedtherein, wherein the metal nano-particles are formed with thermaldissociation of an oxide of the same metal. The gate is disposed on thecharge-trapping layer, and the source/drain is located in the substratebeside the gate.

The method for fabricating a non-volatile memory cell of this inventionis described as follows. A first insulating layer, a metal oxide layerand a second insulating layer are sequentially formed on a substrate. Anannealing is performed to convert the metal oxide layer to a pluralityof metal nano-particles with thermal dissociation, while the firstinsulating layer, the second insulating layer and the metalnano-particles together constitute a charge-trapping layer. A gate isformed on the charge-trapping layer, and then a source/drain is formedin the substrate beside the gate.

Moreover, in the method for fabricating a non-volatile memory cell ofthis invention, several metal oxide layers and insulating layers can bealternately formed on the second insulating layer after the secondinsulating layer is formed. The metal oxide layers will be converted tomulti layers of metal nano-particles in the subsequent annealing step.

As mentioned above, the metal oxide layer(s) is (are) converted to oneor multi layers of metal nano-particles with thermal dissociation inthis invention. Since the metal nano-particles capable of storingcharges are isolated from each other, two bits can be stored in onenon-volatile memory cell of this invention as in the case of theconventional SONOS memory device. Meanwhile, the uniformity of thenano-particles in this invention is better as compared with the priorart, and the electrical properties of the non-volatile memory can bewell controlled.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIGS. 1-3 illustrate a process flow of fabricating a non-volatile memorycell in a cross-sectional view according to a preferred embodiment ofthis invention, wherein FIG. 3 illustrates the structure of thenon-volatile memory cell.

FIGS. 4-5 illustrates a method for forming multi layers of metalnano-particles according to the preferred embodiment of this invention.

FIG. 6 illustrates a programming operation of the non-volatile memorycell illustrated in FIG. 3.

FIG. 7 illustrates a reading operation of the non-volatile memory cellillustrated in FIG. 3.

FIG. 8 illustrates an erasing operation of the non-volatile memory cellillustrated in FIG. 3.

DETAILED DESCRIPTION

FIG. 3 illustrates the structure of a non-volatile memory cell in across-sectional view according to the preferred embodiment of thisinvention.

Referring to FIG. 3, the non-volatile memory cell comprises a substrate100, a charge-trapping layer 140, a gate 150, and a source/drain 160.The substrate 100 is, for example, a p-type silicon substrate. Thecharge-trapping layer 140 is disposed on the substrate 100, comprisingan insulating layer 110 and metal nano-particles 130 contained therein.The insulating layer 110 may comprise silicon oxide, Al₂O₃, HfO₂ orZrO₂, while silicon oxide is more preferable. The metal nano-particles130 are formed with thermal dissociation of an oxide of the same metal,and may comprise platinum (Pt), iridium (Ir), ruthenium (Ru) or hafnium(Hf), while platinum is more preferable because the temperature forthermal dissociation of platinum oxide is lower. The diameter of themetal nano-particles 130 ranges from 1 nm to 20 nm, and the distancebetween two metal nano-particles 130 ranges from 1 nm to 10 nm. The gate150 is disposed on the charge-trapping layer 140, and comprises amaterial such as doped polysilicon. The source/drain 160 is located inthe substrate 100 beside the gate 150, and is doped with an n-typedopant such as phosphorous (P) or arsenic (As).

FIGS. 1-3 illustrate a process flow of fabricating a non-volatile memorycell in a cross-sectional view according to the preferred embodiment ofthis invention.

Referring to FIG. 1, a first insulating layer 110 a, a metal oxide layer120 and a second insulating layer 110 b are sequentially formed on asubstrate 100. The first insulating layer 110 a is formed with a methodsuch as reactive sputtering or thermal oxidation, the metal oxide layer120 with a method such as reactive sputtering, and the second insulatinglayer 110 b with a method such as reactive sputtering or chemical vapordeposition (CVD). The first and the second insulating layers 110 a and110 b may comprise the same material or different materials, and each ofthem comprises, for example, silicon oxide, Al₂O₃, HfO₂ or ZrO₂, whilesilicon oxide is more preferable. The metal oxide layer 120 comprises amaterial such as platinum oxide (PtO_(x)), iridium oxide, rutheniumoxide or hafnium oxide, while PtO_(x) (x=0.1-10) is more preferablebecause the temperature for thermal dissociation of PtO_(x) is lower.

As the first/second insulating layer 110 a/b comprises silicon oxideformed with reactive sputtering, a silicon target is used with argon andoxygen gas introduced. As the metal oxide layer 120 comprises PtO_(x)formed with reactive sputtering, a platinum target is used with argonand oxygen gas introduced, and the sputtering conditions are analogousas in the case of silicon oxide sputtering. For example, the substratetemperature is room temperature, the flow rates of argon and oxygen gasare 5 sccm and 15 sccm, respectively, and the sputtering pressure is 20mTorr for both the SiO_(x) deposition and the PtO_(x) deposition, whilethe sputtering rate for depositing SiO_(x) is 3 nm/min, and that fordepositing PtO_(x) is 2 nm/min.

Referring to FIG. 2, an annealing is performed to convert the metaloxide layer 120 to a plurality of metal nano-particles 130 with thermaldissociation. The metal nano-particles 130 and the first and the secondinsulating layers 110 a and 110 b (insulating layer 110) togetherconstitute a charge-trapping layer 140. The annealing is preferablyunder vacuum for preventing contamination. As the metal oxide layer 120comprises PtO_(x), the annealing can be conducted at 420° C. for 60minutes, for example. The diameter of the platinum nano-particles formedwith thermal dissociation ranges from about 1 nm to about 20 nm, and thedistance between two platinum nano-particles ranges from about 1 nm toabout 10 nm.

Referring to FIG. 3, a gate 150 is then formed on the charge-trappinglayer 140, and the charge-trapping layer 140 is also patterned after thegate 150 is finished, wherein the gate 150 comprises a material such asdoped polysilicon. Thereafter, a source/drain 160 is formed in thesubstrate 100 beside the gate 150 with a method such as ionimplantation. The source/drain 160 is doped with phosphorous (P) orarsenic (As).

Besides, the metal nano-particles in the insulating layer can be formedwith multi layers for increasing the number of trapping sites or otherpurposes. A method for forming multi layers of metal nano-particles isillustrated in FIGS. 4-5.

Referring to FIGS. 4-5, several insulating layers 410 and metal oxidelayers 420 are alternately formed on a substrate 400 to constitute amulti-layer structure. Thereafter, an annealing is performed under thesame conditions as mentioned above, and the metal oxide layers 420 areconverted to multi layers of metal nano-particles 430 with thermaldissociation. The multi layers of metal nano-particles 430 and theinsulating layers 410 together constitute a charge-trapping layer 440.

Operations of the Non-Volatile Memory Cell

FIGS. 6, 7 and 8 illustrate a programming operation, a reading operationand an erasing operation, respectively, of the non-volatile memory cellillustrated in FIG. 3.

Referring to FIG. 6, the non-volatile memory may be programmed withchannel hot electron injection (CHEI) mechanism. As bit 1 is to bewritten to the memory cell, the gate 150 is applied with a high positivevoltage, the source/drain 160 a is grounded, and the source/drain 160 bis applied with a sufficiently high voltage capable of inducing hotelectrons in the channel near the source/drain 160 b. The hot electronsare driven into the right edge of the charge-trapping layer 140 by thehigh positive voltage on the gate 150, and trapped in the metalnano-particles 130 therein. The trapped electrons will not move to theother metal nano-particles 130 because of the isolation of theinsulating layer 110. Analogously, bit 2 (not shown) can be written tothe left edge of the charge-trapping layer 140 and trapped in the metalnano-particles 130 therein by grounding the source/drain 160 b andapplying a sufficiently high voltage to the source/drain 160 a.

Referring to FIG. 7, a bit in the memory cell can be read in the reversedirection. That is, as bit 1 is to be read, the source/drain 160 a isapplied with a positive voltage and the source/drain 160 b is grounded,so that the current in the channel flows in a reverse direction ascompared with the case of programming bit 1. If there are electronsstored in bit 1, as shown in FIG. 7, the portion of the channel underbit 1 is not turned on, and the current in the channel is in a lowlevel. If bit 1 has not been written (not shown), the portion of thechannel under bit 1 can be turned on, and the current in the channel isin a high level. Analogously, bit 2 can be read in a direction that isreverse to the current direction in the programming operation thereof.

Referring to FIG. 8, the non-volatile memory cell may be erased withFowler-Nordheim tunneling mechanism. In the erasing operation, the gate150 is applied with a high negative voltage, and the substrate 100 isgrounded or applied with a positive voltage. With the electric fieldestablished between the gate 150 and the substrate 100, the electronsstored in both bit 1 and bit 2 are driven to the substrate 100 from thecharge-trapping layer 140, so that the erasing operation is completed.

As mentioned above, the metal oxide layer is converted to metalnano-particles with thermal dissociation in this invention. Since themetal nano-particles capable of storing charges are isolated from eachother, two bits can be stored in one non-volatile memory cell as in thecase of the conventional SONOS memory. Meanwhile, the uniformity of thenano-particles in this invention is better as compared with the priorart, and the electrical properties of the non-volatile memory can bewell controlled.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncovers modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A method for fabricating a non-volatile memory cell, comprising:providing a substrate; sequentially forming a first insulating layer, ametal oxide layer and a second insulating layer on the substrate;performing an annealing to convert the metal oxide layer to a pluralityof metal nano-particles with thermal dissociation, while the firstinsulating layer, the second insulating layer and the metalnano-particles together constitute a charge-trapping layer; forming agate on the charge-trapping layer; and forming a source/drain in thesubstrate beside the gate.
 2. The method of claim 1, further comprisingalternately forming several metal oxide layers and insulating layers onthe second insulating layer after the second insulating layer is formed,so that multi layers of metal nano-particles are formed with theannealing.
 3. The method of claim 1, wherein the annealing is conductedunder vacuum.
 4. The method of claim 1, wherein the metal oxide layercomprises platinum oxide (PtO_(x)) and the metal nano-particles compriseplatinum, wherein x ranges from 0.1 to
 10. 5. The method of claim 4,wherein the metal oxide layer is formed by performing a reactivesputtering process with a platinum target and an oxygen flow.
 6. Themethod of claim 5, wherein the reactive sputtering process is conductedunder room temperature.
 7. The method of claim 6, wherein a flow rate ofthe oxygen flow is 15 sccm, a sputtering pressure is 20 mTorr, and asputtering rate is 2 nm/mm.
 8. The method of claim 4, wherein theannealing is conducted at about 420° C. for about 60 minutes.
 9. Themethod of claim 1, wherein a diameter of the metal nano-particles rangesfrom about 1 nm to about 20 nm.
 10. The method of claim 1, wherein adistance between two metal nano-particles ranges from about 1 nm toabout 10 nm.
 11. The method of claim 1, wherein the first and the secondinsulating layers comprise different materials.
 12. The method of claim1, wherein the first and the second insulating layers comprise the samematerial.
 13. The method of claim 12, wherein the first and the secondinsulating layers both comprise silicon oxide.
 14. The method of claim1, wherein the metal oxide layer comprises iridium oxide, rutheniumoxide or hafnium oxide.
 15. The method of claim 1, wherein the first andthe second insulating layers comprise Al₂O₃, HfO₂ or ZrO₂.